Commonly, a steel wire is produced by conducting wire-drawing so as to have a predetermined wire diameter and strength by using a steel wire rod which is produced by hot rolling and patenting treatment conducted as necessary. At a stage of a steel wire rod, when the steel wire rod has low strength, work strain should increase in order to be work-hardened to a predetermined strength during wire-drawing. As a result, a steel wire produced by the wire-drawing has poor ductility. In a case where the steel wire has poor ductility, when the steel wire is torsionally deformed, longitudinal cracking which is called as delamination may occur along a wire-drawing direction of the steel wire at an initial stage of deformation. Once the delamination occurs, stress may be concentrated at a site where the delamination occurs, and fracture of the steel wire may be finally promoted. In order to obtain a steel wire with high strength and excellent ductility by suppressing the occurrence of the delamination in the steel wire, the steel wire rod needs to have high strength and excellent ductility at a stage before the wire-drawing.
Generally, it is known that, when grain size is refined, strength is improved. Similarly, reduction of area (RA) that is an index of ductility of the steel wire rod also depends on austenite grain size. When the austenite grain size is refined, the reduction of area is also improved. Therefore, the austenite grain size of the steel wire rod is to be refined by using carbides or nitrides of Nb, B, and the like as pinning particles.
For example, Patent Document 1 suggests a steel wire rod in which at least one selected from a group consisting of, by mass %, 0.01% to 0.1% of Nb, 0.05% to 0.1% of Zr, and 0.02% to 0.5% of Mo is contained in a high carbon steel wire rod.
In addition, Patent Document 2 suggests a steel wire rod in which the austenite grain size is refined by containing NbC in a high-carbon steel wire rod.
However, in the steel wire rod disclosed in Patent Document 1 and Patent Document 2, expensive elements such as Nb are added, and thus the production cost may increase. Furthermore, since Nb forms coarse carbides and nitrides, these may act as fracture origin, and thus ductility of the steel wire rod may decrease.
Patent Document 3 suggests a method of producing a steel wire rod having high strength and large reduction of area by applying a direct patenting treatment (DLP: Direct in-Line Patenting) without using the expensive elements such as Nb.
In fact, the steel wire rod according to the production method disclosed in Patent Document 3 obtains high strength and large reduction of area without adding the expensive elements. However, at the present time, further improvement in strength and ductility is required. In Patent Document 3, as described in examples thereof, in a case of ensuring tensile strength (TS) of 1200 MPa or more, the reduction of area is less than 45%.
In order to improve properties of the prestressed concrete wire, the zinc-coated steel wire, the spring steel wire, the bridge cable, and the like in which the steel wire rod is used as the materials, it is effective to reduce the diameter of the steel wire rod as small as possible. Since reduction during the wire-drawing is controlled to be small by wire-drawing the steel wire rod with small diameter, the wire-drawn steel wire is controlled to excellent ductility. As a result, the occurrence of the delamination in the steel wire is suppressed. Accordingly, the steel wire rod having the small diameter, high strength, and excellent ductility (that is, large reduction of area) has been anticipated. Specifically, in a case where the diameter is 10 mm or less, a steel wire rod having the tensile strength of 1200 MPa or more and the reduction of area of 45% or more has been anticipated.